My quick description how I wrestled with Shimano bottom bracket in order to replace bearings rather than spending money for new cups/bearings set.
The story began with BB-4600 which seems to be fine for 6 months, and then suddenly, at one evening after ride I realised that whole drivetrain is not spinning freely as it should. Quick inspection revealed that component which apparently should be quite robust died quietly. Left bearing in bottom bracked spins but with grinding feeling. After quick calculations was obvious that I need another one in order to keep my wheels rolling. Eventually decided to make a small experiment. I decided to replace bearings in old cups, and slightly upgrade them in the same time.
Step 1: Find Decent Bearing!
The first step was to find out what size of bearing I'll need. After quick measuring/googling I realised that Shimano makes life hard for those who wants replace just a bearings. The problem is simple. Almost impossible to buy (decent quality) bearing in size 25x37x6mm, but it's broadly available bearing 6805-2RS (sometimes known as 6805-RD) which is 25x37x7mm. I've decided to buy set of ceramic bearings from China trough ebay, as I wasn't sure about result of the whole operation. They cost me £20 including postage so not too bad price after all for (semi)ceramic bearings. I've clicked "Pay" in PayPal and the fun begun.
Step 2: Dissassemble
Viva! Viva! La Revolución!
The cups are tricky to disassemble (actually stays "DO NOT DISASSEMBLE" on the cups). Shimano made the same size of hole inside cups as the internal diameter of bearings. So basically is a small gap (about 1mm) where you can actually attach bearing puller and disassemble it. Sounds almost impossible with standard bearing puller, so I came across with diy bearing puller made from cut washers, screw and other bits and pieces. Two washers on a screw and in between is another washer cut in a half. A sort of sandwich. The images shows only one piece slid.
So I've put the sandwich on the screw and inside move them aside so they catch the bearing and added a nut on top to keep everything compact. I must admit this is quite tricky to set it up, but once is done all you need is a vice or piece of tube as a spacer (or whatever suitable) and you can slowly pull bearing out turning the top nut.
After wrestling with diy puller for nearly an hour I had both disassembled. Pressing new bearings back was quickest and easiest part. I've put the bearings to freezer (old trick in order to use less force pressing them), and in the meantime cleaned cups, grease surfaces where new one will slide.
Step 3: Asemble
On the pictures you can see my "press" made of 8mm screw plus washers and some 19inch rack foot I've found at work, which apparently was exactly the same dimension as outer bearing race. Another hint. Instead of my "special rack foot" you can use socket 36mm from ratchet set. In fact is handy for any bearing as sockets in 1mm steps so is pretty easy to find right dimension. You need to remember that pressure needs to be applied on outer race of the bearing otherwise bearing might be damaged.
Step 4: Final Touch - Seal
Last bit is butchering slightly plastic washers. I've cut them by scissors and sanded down flanges as much as I could.
Made covers from 5.25 inch diskette (do you remember them?! ;-) ) you can see on the pictures. They're probably no necessary but I've decided to have additional protection at least against sand.
The final product you can see on the last image, and I assume if you attempting to do it there's no need explain how to screw them again to the frame ;-)